(1). Field of the Invention
The present invention relates to an optical recording medium and, more particularly, to an optical recording medium that allows recording and/or reproducing information at wavelengths of plural laser beams.
(2). Description of the Related Art
Compact disc-recordables (CD-Rs) are disclosed in, for example, Optical Data Storage 1989, Technical Digest Series Vol. 1, p. 45 (1989) as a write-once type optical recording medium satisfying compact disc (hereinafter, referred to as CD) standards and is on the market. A CD-R is laminated, as shown in FIG. 1, with a recording layer 2, a reflective layer 3 and a protective layer 4 in this order on a transparent resin substrate 1. The recording layer changes physically or chemically when irradiated with a high power laser beam, and thus the information is recorded in the form of pits. The information in the pits can be reproduced by means of irradiating a low power laser beam to the pits formed in the recording layer and detecting a change of a reflectance. A near infrared semiconductor laser having a wavelength of 770 to 830 nm is typically used for recording and reproducing such optical recording media which satisfies CD standards such as the Red Book Standards and the Orange Book standards. Therefore, it is compatible with CD players and CD-ROM players.
Such conventional optical recording media has, however, a recording capacity of only about 650 MB, which is not enough when taking account of recording of dynamic digital images. Thus, there is an increasing demand for an information recording medium having a higher density and a higher capacity.
Recently, a red semiconductor laser having a selected wavelength of from 620 to 690 nm has been developed. Under such circumstances, it is becoming increasingly possible to record and/or reproduce information at a higher density. With this semiconductor laser, such high-density recording media have been developed that have a recording capacity of 5-8 times as large as that of the conventional media. In addition, players for such high density recording media have also been developed. For example, digital video discs (DVD) have been on the market that have dynamic images of 2 hours or longer recorded thereon in a digital format.
In addition, lasers having a wavelength of around 530 nm and 420 nm have been put into practice that uses a higher harmonic wave conversion of a YAG laser. Furthermore, semiconductor lasers having a wavelength of around 490 nm and 410 nm have also been developed.
However, software and continuity of the recorded data are lost when the optical recording medium has no compatibility for various wavelengths even if the high-density optical recording media and players that use such a short-wave laser are introduced. The CD-R media now available on the market have a small reflectance of 10% or lower when it is attempted to reproduce information thereon by using a light beam having a wavelength selected from the range between 400 and 440 nm, between 480 and 540 nm, and between 620 and 690 nm. In addition, a degree of modulation is also very small. The recording is low-to-high recording in which the reflectance of the recorded portion is higher than that of the non-recorded portion. This is not preferable because the polarity is reversed from common CDs (high-to-low recording). Furthermore, a large deformation is observed in recorded waveforms, so that it is difficult to play such a recording medium by using a player for high-density recording which is provided with a laser having a wavelength selected from the range between 400 and 440 nm, between 480 and 540 nm, and between 620 and 690 nm.
Japanese Patent Laid-Open No. 3-290835 discloses a medium having an interference layer comprising a low molecular weight organic compound between a recording layer and a reflective layer comprising an aluminum alloy. This medium uses the aluminum alloy instead of expensive gold for the reflective layer, and has an interference layer to achieve the reflectance of 70% or higher at 780 nm. In this medium, the reflectance is certainly 70% or higher at 780 nm. However, the reflectance is low to the light beam having a wavelength selected from the range between 400 and 690 nm, so that it is difficult to reproduce the recorded information by using a player for a high-density medium.
Japanese Patent Laid-Open Nos. 8-99467 and 8-108623 propose an optical recording medium in which a dye-containing recording layer and a dye-containing light interference layer are laminated. However, these media proposed in the above publications are not good in recording characteristics, especially signal quality obtained by a light beam having a relatively short wavelength.
Japanese Patent Laid-Open No. 7-98887 discloses an example of an optical recording medium in which an additive is added to improve the recording characteristics. However, this additive is only added to the recording layer. No additive is added to a layer such as the light interference layer, which is not directly involved in recording, for improving the recording characteristics.